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Once you use "sign up", that creates a developer account using your email address. But until you click the validation link in the email that gets sent to you as a result, you won't be able to sign in (and, of course, that email address is now part of an account, so you can't use it to create a new one).

If you're not seeing the validation email, you'll want to check your spam folder. Some mail tools are overly aggressive about what gets sent there.